In general, the purpose of marking an object and identifying the object by a marking is to enable an observer to determine his or her affiliation with the object. Objects are marked and identified in many contexts. In the context of fighting between groups, it is important that each group (the observer) be able to distinguish its forces (the objects) from the forces of another group or groups. Identification of one's own forces is particularly important in averting battlefield fratricide. Present methods of marking and identifying forces are problematic.
A problem in identifying forces is that it is necessary to make the identification from as great a distance as possible, without having to come into close contact with unknown forces. Reliable identification based upon normal human vision or magnified vision such as binoculars requires a proximity to objects that may be undesirable. Long-range identification is essential to effectively use long-range weaponry. In battlefield situations, it is also necessary that identification be accomplished as quickly as possible. However, rapid reliable identification is a problem generally, and is an even greater problem in long-range identification. Identification of forces is more difficult under conditions of low or no lighting and periods of unfavorable environmental conditions such as dust that can obscure markings. An additional problem is that if a group identifies its forces by markings which the group places upon the forces, marking must be done in a manner that cannot be easily simulated or abrogated by the enemy.
Certain substances emit definitive spectra of electromagnetic radiation when they are irradiated with infrared electromagnetic radiation. In these substances, emitted radiation is of a wavelength which is shorter than the wavelength of the irradiating radiation. This property can be referred to as “up-conversion,” or “up-converting” of the radiation. Examples of up-conversion materials are compounds such as CaF2, BaF2, CdF2, LaF2 or SrF2 doped with the rare-earth element erbium. It can be appreciated that it would be useful to have a method for marking and identifying objects that utilizes the up-conversion property of materials such as erbium-doped CaF2, BaF2, CdF2, LaF3, or SrF2.